Clearance grinding machine

ABSTRACT

An automatic clearance grinding machine for helically fluted drills wherein a pair of opposed grinding wheels are arranged to simultaneously grind opposed lands of the drill. One grinding wheel is mounted with its rotational axis spaced above the drill axis by a vertical distance substantially equal to one-half the vertical distance between the drill axis and a pivoted diamond point used for dressing the wheel. The other grinding wheel is mounted with its rotational axis spaced below the drill axis by a distance substantially equal to one-half the vertical distance between the drill axis and a second pivoted diamond point used to dress the other wheel. Both diamond dressers are mounted at a location fixed relative to the drill axis. The two grinding wheels are mounted on respective horizontal slides. During a dressing operation, the slides move the grinding wheel axes along respective horizontal paths located substantially midway between the drill axis and the dresser associated with the respective wheel.

United States Patent Kushigian [54] CLEARANCE GRINDING MACHINE Anthony Ktshiglan, P.O. Box SI2, 25804 Novi Road, l'wlorthville` Mich. 48050 [22] Filed: Jan. Il, 1971 [2l] Appl. No.: 105,182

[72] Inventor:

Primary` Examiner- Harold DA Whitehead Attorney-Eames, Kisselle, Raisch & Choate i451 July 1l, 1972 [57] ABSTRACT An automatic clearance grinding machine for helcally uted drills wherein a pair of opposed grinding wheels are arranged to simultaneously grind opposed lands of the drill. One grinding wheel is mounted with iB rotational axis spaced above the drill axis by a vertical distance substantially equal to one-hall` the vertical distance between the drill axis and a pivoted diamond point used for dressing the wheel. The other grinding wheel is mounted with its rotational axis spaced below the drill axis by a distance substantially equal to one-half the vertical distance between the drill axis and a second pivoted diamond point used to dress the other wheel. Both diamond dressers are mounted at a location fixed relative to the drill axis. The two grinding wheels are mounted on respective horizontal slides. During a dresing operation, the slides move the grinding wheel axes along respective horizontal paths located substantially midway between the drill axis and the dresser associated with the respective wheel.

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PTNTDJULI 1 ma SHEET 1 DF 3 ATTORNEYS PTNTUJUL 11 ma 3, 675.372 SHEET a of 3 INVENTORS ANTHONY KUSHIGiAN ATTORNEYS PT'N'T'EDJUL 1 1 |912 3.675 372 SHEU 3 of 3 INVENTOR ANTHONY KUSHIG IAN 45 BY mmmfmzz ATTORNEYS CLEARANCE GRINDING MACHINE In one type of conventional automatic clearance grinder for fluted drills, opposed lands of the drill are ground simultaneously by respective grinding wheels. The grinding wheels are mounted on respective slides that are inched forward during dressing operations. A separate dressing diamond is provided for each grinding wheel. The diamonds are also mounted on separate respective slides. Each time the wheel is advanced, the slide for its associated dresser is also advanced to compensate for the wheel movement. This arrangement requires two separate dresser slides and two separate wheel slides, all of which must be very precisely synchronized to assure proper location of the grinding wheels as their diameters are reduced by repeated dressing. This prior art arrangement is complex and expensive if both precision and ruggedness are to be obtained.

Among the objects of the present invention are to provide a grinding machine that overcomes the disadvantages of the prior art clearance grinding machines referred to hereinabove', that is simple in construction and operation; that achieves effective and efficient simultaneous grinding of opposite lands on a fluted drill or the like; that is rugged yet precise and automatically eliminates errors that would otherwise result from unequal movement of grinding wheel slides; that is versatile and easily adjusted for drills of different diameters and for dressing wheels of different diameters; and/or that achieves a compact arrangement of parts with fewer slides, as contrasted to prior art grinding machines, and with only horizontal motion ofthe slides being required.

Further objects, features and advantages of the present invention will become apparent in connection with the following description, the appended claims and the accompanying drawings in which:

FIG. 1 is a simplified front elevational view of the automatic clearance grinder ofthe present invention;

FIG. 2 is a view taken generally on line 2-2 of FIG. l and illustrating in greater detail the construction of one wheel dresser and the mechanism for locating and guiding a drill;

FIG. 3 is a simplified top plan view of the apparatus of FIG. l;

FIG. 4 is a view diagrammatically illustrating the geometry of the drill, one of the grinding wheels and its associated diamond dressing point; and

FIG. 5 is an enlarged view of another dresser used to dress the front face of a dressing wheel.

Referring in greater detail to the drawings, the automatic clearance grinder of the present invention generally comprises a pair of opposed grinding wheels 10, 12 arranged to simultaneously grind respective lands 14, 16 (FIG. 2) of a double helcally fluted drill 18 as the drill is fed longitudinally therebetween along its rotational axis 20. Associated with each of the wheels l0, 12 is a respective dresser 22, 24 for dressing the respective peripheral surfaces 26, 28 of the respective grinding wheels 10, l2. The wheels 10, 12, the dressers 22, 24 and the drill 18 are arranged in a compact assembly of simple construction so as to maintain proper orientation therebetween for effective grinding as the wheels 10, 12 have their diameters reduced by repeated dressing operations.

More particularly, wheel l is joumalled on a slide 30 for rotation about axis 32 and is driven by a belt 36 and a motor 34 mounted on slide 30. Slide 30 is horizontally reciprocally mounted on a stationary way 38 and is driven in precise increments by a drive unit 38 in a direction toward drill 18, toward the le as viewed in FIG. 1, during a dressing operation. Slide 30 also has a downwardly projecting member 40 connected to a hydraulic cylinder 42 mounted on way 38. Piston 42 reciprocates slide 30 through a very short travel into engagement with drill 18, toward the left as viewed in FIG. 1, during grinding and dressing operations and in the opposite direction to allow a new drill to be fed into grinding position. A third dresser 44 is arranged to dress the front radial face 46 of the wheel 10. As shown in FIG. 5, dresser 44 includes an arm 41 pivoted on a stationary base 43. A diamond point 45 is carried on arm 41 so as to sweep the front face 46 of wheel 10 as arm 41 is pivoted upwardly, clockwise as viewed in FIG. l. The axial location of wheel 10 and the position of point 45 can be accurately set to grind the juncture between the margin of the land 16 and the clearance diameter on drill 18.

Similarly, wheel 12 is joumalled on slide 48 for rotation about axis 50 and is dven by motor 52 and belt 54. Slide 48 is horizontally reciprocal on a stationary way 56 and is driven inwardly toward drill 18, toward the right as viewed in FIG. l by a drive unit 58 during a dressing operation. Slide 48 is also arranged to be driven to grinding and dressing positions by means of a cylinder 60 connected to the downwardly projecting member 62 on slide 48. The front face 64 of wheel 12 is dressed by a fourth dresser 66.

The dresser 22 comprises a diamond dressing point 70 adjustably mounted in a holder 72 which in turn is fixed on an ann 74 that is pivoted at 76 on a stationary base 78. The inter face between holder 72 and arm 74 is accurately located relative to the vertical plane through the drill axis 20. Holder 72 can be removed from the arm 74 and the precise location of the diamond point 70 longitudinally in the holder 72 can be set by means of a suitable jig (not shown). By setting the point 70 accurately relative to the interface 80, the point can be accurately located relative to axis 20 and lands I4` 16 when holder 72 is reassembled on arm 74. The arm 74 of dresser 22 is pivoted on base 78 by means of a cylinder 79. Arm 74 is normally pivoted so that point 70 clears wheel I0 during a grinding operation as shown in full lines in F IG. 2. During a dressing operation, cylinder 79 pivots arm 74 between the positions illustrated in full and dotted lines in FIG. 2 to dress surface 26. Similarly, dresser 24 comprises a diamond point 84 adjustably mounted in a holder 86 on an arm 88 which in turn .is pivoted on a stationary base 90 by a pin 92. The interface 94 between holder 86 and arm 88 is accurately located relative to the vertical plane through the drill axis 20 and hence lands 14, 16. Arm 88 is similarly disposed so that point 84 normally clears wheel 12 during grinding operations with the arm 88 being actuated by a cylinder (not shown) during a dressing operation.

The drive unit 38 generally comprises a screw 100 having a double shouldered collar 102 at one end thereof interengaged with the slide 30. Collar 102 permits limited travel of the slide 30 relative to the collar 102. Cylinder 42 is actuated to move slide 30 to its inner limit of travel on collar 102 (toward the left in FIG. 1 against the inner shoulder on collar 102) during both dressing and grinding operations and to its outer travel limit against the outer shoulder when drills are fed into grinding position. Screw is threaded in a gear I08 and is keyed in a housing 106 for nonrotatable longitudinal movement therein. Gear 108 is driven by gear 110, drive shaft 112 and an actuator 114. Actuator 114 is a one-way sprag clutch actuated by a cylinder (not shown) to rotate shaft 112 and gears 108, in precise increments. Rotation of gear 108 imparts longitudinal movement to screw 100 and hence horizontal advancement of slide 30 in a direction toward drill 18. Drive unit 58 is similarly constructed and comprises a double shouldered collar 118 on a screw 120 which in turn is keyed in a housing 122 and driven by gears 124, 126. Gear 126 is driven by shaft l 12 so as to cause incremental longitudinal movement at screw in response to incremental rotation of the shaft 1 l2. Movement of slides 30, 48 in opposite horizontal directions in response to rotation of shaft 112 is achieved by providing lefthand threads on screw 100 and right-hand threads on screw 120.

Referring to FIGS. 2 and 3, drills are fed from a hopper 130 into a guide 132 where they are in tum fed to the dressing wheels l0, 12 by a plunger 134 operated by a hydraulic cylinder 135. Drills being fed to the grinding wheels 10, 12 enter a holder fixture designated generally at 136 and comprising a guide sleeve 138 fixedly carried on the fixture body 140. A pivotal dog 142 on a shaft 144 joumalled in the fixture body 140 projects through a slot 146 in the sleeve 138 to positively engage in a flute of the drill during a grinding operation. Dog 142 is located immediately adjacent the grinding wheels 10, 12 shown in broken lines in FIG. 2 so as to positively locate the lands 14, 16 of drill 18 relative to the wheels during a grinding operation. Dog 142 also serves to rotate drill 18 as the drill is pushed through the fixture 136 in a direction from left to right as viewed in FIG. 2 during a grinding operation. A spring biased locator 150 also carried on xture body 140 is disposed to engage in a flute of the drill as the drill is moved into the fixture 136.

During initial setup, the diamond points 70, 84 are set in the manner previously indicated so that for a given diameter drill the dressed surfaces 26, 28 will be accurately located to grind lands 14, 16. Cylinders 42, 60 are actuated to shift slides 30, 48 to their inner travel limits on their respective collars 102, 118; i.e., against the inner shoulders of the collars. Slides 30, 48 are then advanced by actuator 114 and screws 100, 120, while cylinders 42, 60, maintain the slides at their inner travel limits on the collars, until wheels 10, 12 are located for dressing. The diamonds 70, 84 are swung across the surfaces 16, 28 while wheels l0, 12 are rotating. After the wheels are dressed, cylinders 42, 60 shift slides 30, 48 to their outer travel limit on collars 102, 118 so that wheels 10, 12 clear drill 18 as it is fed into sleeve 138. Screws 100, 120 remain fixed until the next dressing. Dressing can be automatically repeated after any desired number of grinding operations until wheels 10, 12 are reduced to the minimum usable diameter.

During a feeding operation, the shank end of drill 18 (the right end as viewed in FlG. 2) first engages the locator 150 urging the same upwardly as the drill moves into the sleeve 138 toward the grinding position illustrated in FIGA 2. Dog 142 is pivoted out of sleeve 138 by rotating shaft 144 by suitable automatic means (not shown) so that the drill is free to move into the sleeve. As the drill is fed into the sleeve, locator 150 will drop into one of the flutes and remain seated in the flute causing the drill to rotate as it is fed to the initial grinding position illustrated in full lines in FIG. 2. The position of locator 150 axially of drill 18 is set so that dog 142 seats in the other flute to accurately and positively locate the drill relative to the wheels 10, 12. ln the event that locator 150 fails to locate in a fiute at the appropriate point of travel of plunger 134, locator 150 will actuate a suitable control circuit causing cylinder 135 to push drill 18 on through the fixture 136 and reject the drill. Assuming that locator 150 properly seats in the flute, a suitable control mechanism is actuated to feed the drill at a slower rate through the fixture during a grinding operation. As the drill is being fed into the fixture, the slides 30, 48 are shifted outwardly from the position illustrated in FIG. 1; and upon locator 150 sensing proper location of the drill 18, cylinders 42, 60 are automatically actuated to move slides 30, 48 inwardly toward the drill 18 and into grinding relationship to the drill as shown in FIG. l. The inner limit of travel is set by the innermost shoulders on collars 102, 118. The le end of sleeve 138 is open at both sides at the location of the grinding wheels l0, 12 so that the grinding wheels can engage the lands 14, 16 on the drill. As drill 18 is fed slowly past wheels l0, 12, the wheels grind the clearance as illustrated in broken lines in FIG. 2.

As indicated earlier, according to an important aspect of the present invention, the geometry of wheels l0, 12, dressers 22, 24 and slides 30, 48 compensates for reductions in wheel diameter due to repeated dressings, as best illustrated in FIG. 4 for the wheel 10. FIG. 4 is intended merely to illustrate the principle of the geometry involved and is not intended to be to scale. As wheel wears, it is periodically advanced by screw 100 to move wheel l0 incrementally inwardly toward the dresser 22. By way of example, an infeed of 0.001 inch for each dressing has been found to be satisfactory for most applications. During a dressing operation, cylinder 42 is actuated to move slide 30 to its left-most limit of travel against the inner shoulder of collar 102 which corresponds to the position of slide 30 during a grinding operation. The travel of slides 30, 48 on collars 102, 118 need only be sufficient to assure that the wheels do not interfere with drill 18 as it is fed into grinding position, for example, a travel of 0.025 inch. With the wheel 10 positioned for dressing as illustrated in FIG. 4, the diamond point 70 is swung across surface 26 at a point of engagement designated at 156. For purposes of illustration, the drill 18 is illustrated in FIG. 4 to show a point of engagement 158 between the grinding surface 26 and the drill 18 during a grinding operation,

The points of engagement 156, 158 form an acute included angle whose vertex is the rotational axis 32 of the grinding wheel 10. lnfeed of slide 32 with repeated dressing operations moves the axis 32 along the horizontal line 160 to a location designated 32'. The line of travel 160 of the axis 32 bisects the angle between the points of engagement 156, 158 regardless of the location of the axis 32 along the line of travel 160. Stated differently, the point of engagement 158 is horizontally in line with the drill axis 20; and the horizontal line of travel of the grinding wheel axis 32 is midwaybetween the drill axis 20 and the point of engagement 156; i.e., the vertical distance between the engagement point 158 and the line of travel 160 is equal to the vertical distance between the line of travel and the engagement point 156. Hence when the axis 32 has moved to the location 32', the location of the points of engagement 156, 158 remains the same for the grinding surface 26' of the reduced diameter wheel.

Once the location of the engagement point 156 is set relative to the axis 20, accurate location of the engagement point 158 after repeated dressing by point 70 is assured regardless of wheel diameter. Since the horizontal position of the diamond point 70 sets the position of the grinding wheel surface with respect to the drill axis 20, it will be apparent that the cleared diameter can be easily adjusted, for example, for drills of different sizes, by proper selection of the longitudinal position of the diamond point 70 in the holder 72. To obtain a larger cleared diameter, for example, for clearing drills of larger diameters, the diamond point 70 is merely set to extend further to the right as viewed in FIG. 1 from the holder 72.

The clearance grinding machine described hereinabove achieves effective automatic grinding with automatic dressing yet is simple in construction, by comparison to prior art grinding machines, and is precise and rugged. Error is eliminated because the dressers 22, 24 are stationarily mounted and do not require separate slides that would otherwise have to be moved in synchronism with the grinding wheel slides. The inner shoulders on collars 102, 118 positively set the same inner limit of travel of the wheels 10, 12 during both dressing and grinding operations. Hence any error that might occur from unequal movement of the slides 30, 48 as the slides are advanced during repeated dressings is automatically eliminated.

lt will be apparent that the clearance grinding machine has been described hereinabove for purposes of illustration and is not intended to indicate limits of the present invention, the scope of which is dened by the following claims.

l claim:

1. Apparatus for simultaneously grinding opposed lands on a rotary cutting tool or the like comprising means for feeding said cutting tool longitudinally along a first axis during a grinding operation, a first grinding wheel rotatable about a second axis that is generally parallel to said first axis and radial of said cutting tool in a first direction, said grinding wheel having a peripheral surface portion adapted to grind one of said lands at a first point of engagement therebetween during a grinding operation, a second grinding wheel rotatable about a third axis that is generally parallel to said first axis and radial of said cutting tool in a second direction generally opposite said first direction, said second grinding wheel having a peripheral surface portion adapted to grind the other of said lands at a second point of` engagement therebetween during a grinding operation, first wheel dressing means for dressing said first grinding surface portion at a third point of engagement therebetween during a dressing operation, said third point of engagement being spaced circumferentially of said first grinding wheel from said first engagement point so that said first and third engagement points and said second axis dene a first angle, second wheel dressing means for dressing said second grinding surface portion at a fourth point of engagement therebetween during a dressing operation, said fourth point of engagement being spaced circumferentially of said second grinding wheel from said second engagement point so that said second and fourth engagement points and said third axis define a second angle, means for moving said first grinding wheel so that said second axis moves toward said first axis in a direction substantially bisecting said first angle and means for moving said second grinding wheel so that said third axis moves toward said first axis in a direction substantially bisecting said second angle.

2. The apparatus set forth in claim l wherein said apparatus further comprises means for rotating said cutting tool about said first axis during a grinding operation.

3. The apparatus set forth in claim 2 wherein said cutting tool has helical flutes and wherein said means for rotating said cutting tool comprises a dog engaged in one of said flutes.

4. The apparatus set forth in claim 3 wherein said dog is disposed longitudinally of said first axis immediately adjacent one of said grinding wheels to positively locate said lands with respect to said grinding wheel surface portions during a grinding operation4 S. The apparatus set forth in claim l wherein said first, second and third axes extend horizontally and are substantially coplanar with each other.

6. The apparatus set forth in claim 5 wherein said first and second grinding wheel portions engage said cutting tool at opposite sides thereof during a grinding operation and wherein said means for moving said first grinding wheel and said means for moving said second grinding wheel are arranged and constructed to move said first and second grinding wheels in opposite horizontal directions toward said first axis.

7. The apparatus set forth in claim 6 wherein said second axis and said third point of engagement are above said first axis, said first angle is an acute angle, said second axis and said fourth point of engagement are below said first axis and said second angle is an acute angle` 8, The apparatus set forth in claim 7 wherein said first grinding wheel is rotatably mounted on a first slide, said means for moving said first grinding wheel comprises first actuator means for moving said slide toward said cutting tool during a dressing operation and second actuator means for moving said slide toward and away from said cutting tool between successive grinding operations so that said first grinding wheel can be withdrawn in a direction away from said first axis while a cutting tool is positioned for grinding and then moved toward said cutting tool into engagement therewith during a grinding operation.

9. The apparatus set forth in claim 8 wherein said first wheel dressing means is pivoted about a horizontal axis that is fixed relative to said first axis and said first and second actuator means move said first slide relative to said first wheel dressing means.

10. The apparatus set forth in claim l wherein said first grinding wheel is rotatably mounted on a first slide, said means for moving said first grinding wheel comprises first actuator means for moving said slide toward said cutting tool during a dressing operation and second actuator means for moving said slide toward and away from said cutting tool between successive grinding operations so that said first grinding wheel can be withdrawn in a direction away from said first axis while a cutting tool is positioned for grinding and then moved toward said cutting tool into engagement therewith during a grinding operation.

l1. The apparatus set forth in claim l0 wherein said first actuator means comprises a screw mounted for non-rotatable longitudinal movement and having a double shouldered collar at one end thereof interengaged with said first slide so that said first slide has a limited travel on said collar between said shoulders, rotatable nut means threaded on said screw and means for rotating said nut to advance said screw and wherein said second actuator means comprises means for shiftin said first slide on said collar between travel limits set by said s oulders.

12. Apparatus for simultaneously grinding opposed lands on a rotary cutting tool or the like comprising means for feeding said cutting tool longitudinally along a first axis during a grinding operation, a first grinding wheel rotatable about a second axis that is generally parallel to said first axis and radial of said cutting tool in a first direction, said grinding wheel being adapted to grind one of said lands at a first point of engagement therebetween, a second grinding wheel rotatable about a third axis that is generally parallel to said first axis and radial of said cutting tool in a second direction generally opposite said first direction, said second grinding wheel having a peripheral surface adapted to grind the other of said lands at a second point of engagement therebetween, first wheel dressing means for dressing said first grinding surface portion at a third point of engagement therebetween, second wheel dressing means for dressing said second grinding surface at a fourth point of engagement, means for moving said first grinding wheel into engagement with said cutting tool so that said second axis moves along a first path of travel that is substan tially midway between said first and third engagement points measured in a direction perpendicular to said first travel path, and means for moving said second grinding wheel into engagement with said cutting tool so that said second axis moves along a second path of travel that is substantially midway between said second and fourth engagement points measured in a direction perpendicular to said second travel path.

13. The apparatus set forth in claim ll wherein said third point of engagement is perpendicularly spaced from said first point of engagement a distance substantially equal to the perpendicular spacing between said second and said fourth points of engagement.

14. The apparatus set forth in claim tl wherein said first, second and third axes extend horizontally and are substantially coplanar, the vertical distance between said first point of engagement and said first path of travel is substantially equal to the vertical distance between said first path of travel and said third point of engagement, and the vertical distance between said second point of engagement and said second path of travel is substantially equal to the vertical distance between said second path of travel and said fourth point of engagement.

l5. Apparatus for simultaneously grinding opposed lands on a rotary cutting tool or the like comprising means for feeding said cutting tool along a first horizontally extending axis during a grinding operation, a first grinding wheel rotatable about a second horizontally extending axis, first wheel dressing means for engaging said first wheel at a location that remains fixed relative to said first axis as the diameter of said first grinding wheel is reduced during repeated dressing operations, means for moving said first grinding wheel toward said first axis and said first dressing means while the position of said first dressing means remains fixed relative to said first axis, a second grinding wheel rotatable about a third horizontally extending axis, second wheel dressing means for engaging said second wheel at a location that remains fixed relative to said first axis as the diameter of said second grinding wheel is reduced during repeated dressing operations, and means for moving said second grinding wheel toward said first axis and said second dressing means while the position of said second dressing means remains fixed relative to said first axis. 

1. Apparatus for simultaneously grinding opposed lands on a rotary cutting tool or the like comprising means for feeding said cutting tool longitudinally along a first axis during a grinding operation, a first grinding wheel rotatable about a second axis that is generally parallel to said first axis and radial of said cutting tool in a first direction, said grinding wheel having a peripheral surface portion adapted to grind one of said lands at a first point of engagement therebetween during a grinding operation, a second grinding wheel rotatable about a third axis that is generally parallel to said first axis and radial of said cutting tool in a second direction generally opposite said first direction, said second grinding wheel having a peripheral surface portion adapted to grind the other of said lands at a second point of engagement therebetween during a grinding operation, first wheel dressing means for dressing said first grinding surface portion at a third point of engagement therebetween during a dressing operation, said third point of engagement being spaced circumferentially of said first grinding wheel from said first engagement point so that said first and third engagement points and said second axis define a first angle, second wheel dressing means for dressing said second grinding surface portion at a fourth point of engagement therebetween during a dressing operation, said fourth point of engagement being spaced circumferentially of said second grinding wheel from said second engagement point so that said second and fourth engagement points and said third axis define a second angle, means for moving said first grinding wheel so that said second axis moves toward said first axis in a direction substantially bisecting said first angle and means for moving said second grinding wheel so that said third axis moves toward said first axis in a direction substantially bisecting said second angle.
 2. The apparatus set forth in claim 1 wherein said apparatus further comprises means for rotating said cutting tool about said first axis during a grinding operation.
 3. The apparatus set forth in claim 2 wherein said cutting tool has helical flutes and wherein said means for rotating said cutting tool comprises a dog engaged in one of said flutes.
 4. The apparatus set forth in claim 3 wherein said dog is disposed longitudinally of said first axis immediately adjacent one of said grinding wheels to positively locate said lands with respect to said grinding wheel surface portions during a grinding operation.
 5. The apparatus set forth in claim 1 wherein said first, second and third axes extend horizontally and are substantially coplanar with each other.
 6. The apparatus set forth in claim 5 wherein said first and second grinding wheel portions engage said cutting tool at opposite sides thereof during a grinding operation and wherein said means for moving said first grinding wheel and said means for moving said second grinding wheel are arranged and constructed to move said first and second grinding wheels in opposite horizontal directions toward said first axis.
 7. The apparatus set forth in claim 6 wherein said second axis and said third point of engagement are above said first axis, said first angle is an acute angle, said second axis and said fourth point of engagement are below said first axis and said second angle is an acute angle.
 8. The apparatus set forth in claim 7 wherein said first grinding wheel is rotatably mounted on a first slide, said means for moving said first grinding wheel comprises first actuator means for moving said slide toward said cutting tool during a dressing operation and second actuator means for moving said slide toward and away from said cutting tool between successive grinding operations so that said first grinding wheel can be withdrawn in a direction away from said first axis while a cutting tool is positioned for grinding and then moved toward said cutting tool into engagement therewith during a grinding operation.
 9. The apparatus set forth in claim 8 wherein said first wheel dressing means is pivoted about a horizontal axis that is fixed relative to said first axis and said first and second actuator means move said first slide relative to said first wheel dressing means.
 10. The apparatus set forth in claim 1 wherein said first grinding wheel is rotatably mounted on a first slide, said means for moving said first grinding wheel comprises first actuator means for moving said slide toward said cutting tool during a dressing operation and second actuator means for moving said slide toward and away from said cutting tool between successive grinding operations so that said first grinding wheel can be withdrawn in a direction away from said first axis while a cutting tool is positioned for grinding and then moved toward said cutting tool into engagement therewith during a grinding operation.
 11. The apparatus set forth in claim 10 wherein said first actuator means comprises a screw mounted for non-rotatable longitudinal movement and having a double shouldered collar at one end thereof interengaged with said first slide so that said first slide has a limited travel on said collar between said shoulders, rotatable nut means threaded on said screw and means for rotating said nut to advance said screw and wherein said second actuator means comprises means for shifting said first slide on said collar between travel limits set by said shoulders.
 12. Apparatus for simultaneously grinding opposed lands on a rotary cutting tool or the like comprising means for feeding said cutting tool longitudinally along a first axis during a grinding operation, a first grinding wheel rotatable about a second axis that is generally parallel to said first axis and radial of said cutting tool in a first direction, said grinding wheel being adapted to grind one of said lands at a first point of engagement therebetween, a second grinding wheel rotatable about a third axis that is generally parallel to said first axis and radial of said cutting tool in a second direction generally opposite said first direction, said second grinding wheel having a peripheral surface adapted to grind the other of said lands at a second point of engagement therebetween, first wheel dressing means for dressing said first grinding surface portion at a third point of engagement therebetween, second wheel dressing means for dressing said second grinding surface at a fourth point of engagement, means for moving said first grinding wheel into engagement with said cutting tool so that said second axis moves along a first path of travel that is substantially midway between said first and third engagement points measured in a direction perpendicular to said first travel path, and means for moving said second grinding wheel into engagement with said cutting tool so that said second axis moves along a second path of travel that is substantially midway between said second and fourth engagement points measured in a direction perpendicular to said second travel path.
 13. The apparatus set forth in claim 11 wherein said third point of engagement is perpendicularly spaced from said first point of engagement a distance substantially equal to the perpendicular spacing between said second and said fourth points of engagement.
 14. The apparatus set forth in claim 11 wherein said first, second and third axes extend horizontally and are substantially coplanar, the vertical distance between said first point of engagement and said first path of travel is substantially equal to the vertical distance between said first path of travel and said third point of engagement, and the vertical distance between said second point of engagement and said second path of travel is substantially equal to the vertical distance between said second path of travel and said fourth point of engagement.
 15. Apparatus for simultaneously grinding opposed lands on a rotary cutting tool or the like comprising means for feeding said cutting tool along a first horizontally extending axis during a grinding operation, a first grinding wheel rotatable about a second horizontally extending axis, first wheel dressing means for engaging said first wheel at a location that remains fixed relative to said first axis as the diameter of said first grinding wheel is reduced during repeated dressing operations, means for moving said first grinding wheel toward said first axis and said first dressing means while the position of said first dressing means remains fixed relative to said first axis, a second grinding wheel rotatable about a third horizontally extending axis, second wheel dressing means for engaging said second wheel at a location that remains fixed relative to said first axis as the diameter of said second grinding wheel is reduced during repeated dressing operations, and means for moving said second grinding wheel toward said first axis and said second dressing means while the position of said second dressing means remains fixed relative to said first axis. 